Set python hash seed to 0 in tox.ini

New tox (>=1.7.0) sets a random python hash seed by default. This is
generally good for testing because it will help keep projects working
regardless of the hash seed, but heat unittests don't currently pass
with a random hash seed so set it to the python default seed.

This change will allow us to use new tox again and remove the
restriction on tox<=1.6.1 to run unittests.

Redundant setenvs for specific test envs are removed to prevent them
from overriding the PYTHONHASHSEED value.

Note this change will need to be backported to the stable branches to
keep unittests there working with new tox as well.

Change-Id: I2370759546adee7bf5e20fa214ed7fa1f31d6bfe
Add `immutable` property attribute

Currently resources support only in-place update or (default)
UpdateReplace. In order to support AWS "Updates are not supported"
feature, this new property attribute is added, with default value False.
It is going to be used for denying an UpdateReplace
behavior of the resource (similar to update_allowed currently).

Change-Id: Ie94241630d9595b58ff6b84bcb94459b0785fc03
Implements: blueprint implement-aws-updates-not-supported
Add APIs to create and manipulate snapshots

This adds a new action on stacks to snapshots them, and specific actions
to show and delete them.

blueprint stack-snapshot
Co-Authored-By: ala.rezmerita@cloudwatt.com

Change-Id: I225b199f3a25a0b40653d654d7fab8cf9de3f242
Merge "Set python hash seed to 0 in tox.ini"
Merge "Add APIs to create and manipulate snapshots"
Merge "Add `immutable` property attribute"